Some thoughts on why your wife likes what she likes:
1. Easy introduction, but deep gameplay. Neverwinter Nights has a lot under the hood, but the gameplay itself is pretty easy point and click. All of popcap's games fall under this umbrella, which is why Plants vs. Zombies is no suprise. Even Demigod has been labeled as a beginner RTS, in that you only have the one character to worry about.
2. More of the carrot. Rewards! Rewards create addictive gameplay. Plants vs. Zombies is loaded with them, as is Neverwinter Nights. As is Demigod.
3. Obvious sticks! Most good games have something good that you're toying around with, and something else you're trying to avoid. All of the games you mention have very obvious sticks - avoid death! Don't let the zombies by!
4. Quick restarts. In Plants Vs. Zombies, if a zombie gets by, you can restart the level very easily. In NWN, quick saves work wonders, and even if you don't have a good save point, you restart at the temple. Demigod makes you sit out for a minute, but then you can game again.
5. Lets face it - they're easier to win at. Once you learn the basics, you can breeze through Plants vs. Zombies with only a couple losses. Depending on the difficulty level, Neverwinter Nights can be the same way. These are either single player, or co-op games, so this goes easily.
The more interesting thing is trying to figure out what she doesn't like in games, and so you know what to avoid. While its hard to gather from just a list of games that "hit" with her, here's my thoughts:
1. How do you play this @#&$*^ game? A lot of the "gamer" games have complex control schemes that require a good bit of getting used to. Sure, FPSes come natural to gamers, but teaching them to run around and shoot something on a map is hard enough. Add in skills and special abilities, and this can get crazy.
2. Help! I'm lost! A lot of FPSes and RPGs require a lot of attention as to where to go. If you're worried about how to walk and shoot, you'll miss the sign that hints at where you are supposed to go. Neverwinter Nights is really direct about what you need to do and where you need to go, and I'd imagine its even easier with a guide there to help.
3. I died again, I don't get it, I can't play this stupid game. I'd tred lightly with Tropico for this reason - there is so much to juggle, it gets very overwhelming. You can lose because of a military coup, the Americans or Soviets kicking you out, you lose an election, or rebels fight and take over. What can make these games worse is that you have to start over when this happens - you lost, try again. Sure, Plants Vs. Zombies does this, but you haven't spent an hour getting to that point just to start over.
4. I'm just no good at this game. Competitive multiplayer games mean that beginners lose well more than half the time, which is not forgiving. It makes it hard to get into and stay enthusiastic about.
Some suggestions for you:
Single Player:
Puzzle Quest. Bejeweled with theme! Easy to pick up and get into, lots of carrots.
Crayon Physics. A lot of fun to play through the levels.
Torchlight - Comes out today online. Should be easy for her to pick up, and will give you a good idea if she'd like to play Diablo 3 with you. If she really digs it, you can grab Titan's Quest for you both.
Peggle. Addictive! Try it out online for free.
Mevo and the Grooveriders - a cute rhythm game.
Rollercoaster Tycoon 3. A great introduction to an economic sim game, hidden under the shiny guise of a theme park.
The Sims/2/3. A great juggling act, following the successful line of thought that if you have a million things you want/need to do, but only so many things you can do, it stays challenging. Lots of carrots with shopping.
Multiplayer:
Audiosurf. Pick her favorite music, run along and pick up colored blocks to score points. Theres no "losing," just low scores and high scores.
Warhammer: Dawn of War 2. What!? That game's insane! Yeah, but the campaign is co-op, and you get new items and level up your squads between missions. Theres also a new last stand mode, where you can play with her and one random chap against wave after wave of oncoming monsters, trying to better your score.
